The continuous casting of thin metal strip has been employed with mixed success. Prior processes for the continuous casting of metal strip have been limited to a relatively small number of alloys and products. It has been found that as the alloy content of various metals are increased, the surface quality of the strip deteriorates. As a result, many alloys must be fabricated using ingot methods.
Relatively pure aluminum product such as foil can be continuously strip cast on a commercial basis. Building products can be continuously strip cast, principally because surface quality in the case of such building products is less critical than in other aluminum products, such as can stock. However, surface quality problems appear as the alloy content of aluminum is increased and strip casting has generally been unsuitable for use in malting many aluminum alloy products.
A number of continuous casting machines have been proposed in the prior art. One conventional device is a twin belt strip casting machine, but such machines have not achieved widespread acceptance in the casting of many metals, especially not with metal alloys with wide freezing ranges. In this twin belt strip casting equipment, two moving belts define a moving mold for the metal to be cast. Many prior art processes cool the belts in the area adjacent to the solidification. However, the belt is subjected to extremely high thermal gradients because molten metal is in contact with one side of the belt and a coolant is in contact with the other side of the belt. The dynamically unstable thermal gradients cause distortion in the belt, and consequently neither the upper nor the lower belt is flat. Consequently, the cast metal strip has areas of segregation and porosity.
There are systems that are much more effective at continuous strip casting because they cool the belts when they are not in contact with the solidifying or solid metal. These devices are shown in the following United States patents, which are all incorporated by reference in their entireties: U.S. Pat. Nos. 5,470,405; 5,514,228; 5,515,908; 5,564,491; 5,496,423; 5,363,902; and 5,356,495. U.S. Pat. No. 5,363,902 shows a cooling system in which a cooling box is placed on the belt when the belt is not in contact with the solidifying or solid metal. However, there still remains a need to produce an apparatus that can cast an alloy with acceptable surface properties using belts that do not have distortion.